Bi-propellant rocket system



April 30, 1968 'r. A. WHATLEY BIPROPELLANT ROCKET SYSTEM 3 Sheets-Sheet 1 Filed Nov. 18, 1964 A a n E m M 0.8.- UCBSU L m m T m E I. m 2 o r m w 7 w m w m w a 9. wmafimaz WEIGHT PERCENT ETHANE FIG. I

April 30, 1968 T. A. WHATLEY BI-PROPELLANT ROCKET SYSTEM 3 Sheets-Sheet 2 Filed Nov. l8,' 1964 uow 9523 w PROPANE METHANE 0.. I wmDh mmazwk WEIGHT PERCENT PROPANE FIG. 2

INVENTOR. Thomas A Wharley gww April 30, 1968 T. A. WHATLEY BI-PROPELLANT ROCKET SYSTEM INVENTOR 5 t e e h w W h I s 3 6 I w. a 9 1 8 1 w N d e 1 1 F THOMAS A. WHATLEY United States Patent 3,380,250 BI-PROPELLANT ROCKET SYSTEM Thomas A. Whatley, Santa Clara, Calif, assignor to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Filed Nov. 18, 1964, Ser. No. 412,209 Claims. (Cl. 60-204) The present invention relates to a liquid propulsion system and more particularly to a bi-propellant, liquid propulsion system wherein the melting point of the liquid fuel is selectively adjusted to be compatible with cryogenic oxidizers.

A variety of space missions are currently being considered which will require long-residence-times in space. Because of their versatility and high performance, liquid propulsion systems are considered to be highly suited for such missions, the requisite conditions being that the oxidizer and fuel provide a relatively high specific impulse to the system and have a substantial overlapping liquid range in order to maintain both propellants in a liquid state at the same temperature.

It has been found that if propellants are used in the propulsion system which do not have overlapping liquid ranges, the tanks containing the respective propellants must be insulated from one another or else heat transfer between the two propellant tanks will cause undesirable phase changes in the propellants, such as volatilization or freezing. Such insulation of the propellant tanks results in unnecessary bulk and weight which reduces the payload of the system.

Presently, fluorine and various mixtures of oxygen and fluorine are being considered as liquid oxidizers for space missions because of their high performance capabilities. Although hydrogen would be the highest performance fuel to use with these oxidizers, its low density and very low boiling point substantially diminish its space storability. It has been found, however, that various low molecular weight hydrocarbons make excellent fuels for use with these oxidizers, but the melting points of the most desirable hydrocarbons, methane and ethane, are invariably higher than the boiling point of fluorine and preferred mixtures of fluorine and oxygen. Consequently, if the temperature of the system is maintained so that the hydrocarbon fuel is in a liquid state, the oxidizer, at the same temperature, will tend to volatilize. It is undesirable to attempt to maintain a cryogenic propellant at any temperature significantly above its normal boiling temperature. This is because an increase in temperature causes a marked decrease in density, effectively reducing tankage capacity, and, in addition, causes a rapid rise in vapor pressure, necessitating stronger and heavier tankage. In the usual case, tank temperature is controlled by constant pressure venting whereby cooling is effected by the latent heat of vaporization of the propellant. An increase in tank temperature renders this process less efiicient and can increase boil-off rate since heat of vaporization decreases with temperature, becoming zero at the critical temperature. Thus cryogenic propellants are most efiiciently stored at or below their normal boiling temperatures.

It is unexpectedly discovered that certain hydrocarbons form eutectic compositions which have a melting point substantially below the boiling point of fluorine, preferred fluorine-oxygen mixtures and liquid nitrogen, the conventional coolant for cryogenic oxidizers. Accordingly, by using the preferred fuels of the present invention with fluorine or a fluorine-oxygen mixture (FLOX), space systems can be devised wherein the two propellants may be placed in thermal contact without danger of one phase freezing or volatilizing; and in addition, sys- 3,380,250 Patented Apr. 30, 1068 tems employing liquid nitrogen to maintain the oxidizer in a liquid state do not run the risk of freezing the fuel.

It is an object, tehrefore, of the present invention to provide high performance, bi-propellant rocket systems using liquid oxidizers and liquid fuels which have substantially overlapping liquid ranges.

It is yet another object of the present invention to provide liquid fuels which are compatible for use with fluorine and mixtures of oxygen and fluorine.

It is another object to provide fuels which do not freeze at temperatures above the boiling point of liquid nitrogen.

It is a still further object of the present invention to provide hydrocarbon fuels which are thermally compatible with high-efliciency, cryogenic oxidizers.

These together with various ancillary objects and features of the invention Will become more readily apparent upon consideration of the teachings as hereinbelow set forth in detail and illustrated in the accompanying drawings wherein:

FIG. 1 is an equilibrium phase diagram of methane and ethane.

FIG. 2 is an equilibrium phase diagram of methane and propane.

FIG. 3 is a schematic representation partly in section of a rocket motor according to this invention.

In FIG. 1 a phase diagram is shown for methane and ethane at pressures which are saturation vapor pressures. Superimposed thereon are the boiling points of fluorine, nitrogen and a 7030 mixture of fluorine and oxygen. It will be noted from the phase diagram that ethane has a melting point of approximately 183 C. Hence, when ethane is used with the 70-30 mixture of fluorine and oxygen in a bi-propellant motor, there is a hiatus of over four degrees between the liquid ranges of the two propellants and consequently, it is impossible to design a system which will maintain both propellants in a liquid state at the same temperature.

On the other hand, methane is seen to have a melting point of approximately 183.2 C. and consequently, it, for the same reasons as set forth for ethane, cannot be efficiently used with a 70-30 mixture of fluorine and oxygen in a long-residence-time space vehicle. As pointed out on page 318 of the American Institute of Chemical Engineering Journal of September 1958, it has been previously thought that the melting points of mixtures of methane and ethane would merely fall on a straight line between the two melting points. However, it has been discovered in the present invention that, in fact, a eutectic composition is formed at approximately 55 percent methane with 45 percent ethane, which melting point is at approximately 199 C. This melting point is substantially below the boiling point of 186.9 C. for the 7030 mixture of fluorine and oxygen, providing thereby an overlap of 12 degrees in the liquid ranges of the two propellants. By employing appropriate radiant energy reflecting and absorbing means on the surface of the space vehicle, the temperature and the propellant storage area may be maintained within this 12 degree range, thereby maintaining both propellants in a liquid state. The 12 centigrade overlap in liquid ranges is broad enough to pose no difliculties in system design whereby the temperature range may be maintained without difliculty. With accurate temperature control means, it is possible to use other than eutectic mixtures and, as it can be seen from FIG. 1, methane may be adjusted from 7 to percent when used with 7030 FLOX, or from 9 to 87 percent when used with liquid fluorine.

Referring now to FIG. 2, it will be seen that substantially the same eitect is created by using mixtures of propane and methane. Here a mixture of 30.3 percent of methane and 69.7 percent propane produces a eutectic having a melting point of 198 C. Although approximately eutectic compositions are preferred to permit ready maintenance of both propellants in a liquid state, it is evident from FIG. 2 that propane in excess of 12 percent will provide a propellant with a melting point less than the boiling point of 70-30 FLOX. However, where a liquid system is being designed to employ an oxidizer which must be maintained in a liquid state by means of liquid nitrogen, the fuel mixture of propane and methane will be liquid only where the propane content is from approximately 52 percent to 82 percent.

It is an important aspect, therefore, of the present invention that no other fuels are presently known, save hydrogen, which are liquid below the boiling point of nitrogen. The present invention, however, has discovered two such fuels. One is a mixture of from approximately 52 to 82 percent by weight of propane in methane, and the other is a mixture of from approximately 32 to 68 percent by weight of ethane in methane. Both of these hydrocarbon fuel systems are highly energetic and have none of the serious storability problems possessed by hydrogen as a fuel. Consequently, the present invention makes available fuels which are compatible to high-efficiency, cryogenic oxidizers which must be held at low temperatures by the conventional cryogenic medium, nitrogen.

Although 70-30 mixtures of fluorine and oxygen have been shown as a preferred oxygen-fluorine mixture, it will be apparent that mixtures of 50-50 fluorine and oxygen, mixtures of 35-65 fluorine and oxygen as well as many other oxidizers are suitable for use with fuels of the present invention.

It is also apparent that the ternary mixture of methane, propane and ethane will exhibit freezing points desirable for the purposes of the present invention.

Referring now to FIG. 3, a schematic representation of a liquid bipropellant rocket system according to this invention is shown. Such a system comprises a thrust chamber with associated conventional feed lines 12 and 13, control valves 14 and 15 and pumps and 21 which are supplied with the fuel and oxidizers described above from tank 11 which is divided into a fuel compartment 16 and oxidizer compartment 17 by partition 12. Pressure relief valves 18 and 19 are associated with each compartment to provide for cooling of the fuel and oxidizer by the latent heat of vaporization of the fuel and oxidizer. As an alternative cooling means, liquid nitrogen could be circulated by means 22 around tank 11 according to this invention without freezing the propellants. i

From the aforementioned examples, it is clear, therefore, that various fluorine-oxygen mixtures may be used in combination with fuel systems consisting of mixtures of methane, ethane and propane and accordingly, it will be understood that the aforementioned specific embodiments are descriptive rather than limiting in nature and that various changes, combinations, substitutions or modifications may be employed in accordance with these teachings without departing either in spirit or in scope from this invention in its broader aspects.

I claim:

1. A bi-propellant motor comprising first and second storage tanks containing first and second liquid propellants, a thrust chamber, means for injecting said first and second propellants into said thrust chamber, and means for maintaining said propellants at a predetermined temperature, said temperature being no greater than about 186 C. said first propellant being an oxidizer having a boiling point between about -l86 C. and said predetermined temperature said predetermined temperature and said second propellant being a high-density fuel having a freezing point below said predetermined temperature.

2. The motor of claim 1 wherein said high density fuel is selected from the group consisting of mixtures of methane and ethane and mixtures of methane and propane.

'3. The motor of claim 2 wherein said oxidizer is selected from the group consisting of fluorine and mixtures of oxygen and fluorine.

4. The motor of claim 2 wherein said predetermined temperature is the boiling point of nitrogen and means are provided for passing liquid nitrogen in heat exchange relationship with said storage tanks.

5. In method for operating a cryogenic propellant motor wherein a cryogenic liquid oxidizer and a cryogenic liquid fuel are injected from separate sources into a combustion chamber and reacted therein to produce hot combustion gases, the improvement wherein the sources of said oxidizer and said fuel are maintained at a predetermined temperature no greater than about l86 C.; said oxidizer has a boiling point between about -186 C. and said predetermined temperature, and said fuel is a high density fuel having a freezing point below said predetermined temperature.

6. The method of claim 5 wherein said fuel is selected from the group consisting of mixtures of methane and ethane and mixtures of methane and propane.

7. The method of claim 6 wherein said oxidizer is selected from the group consisting of fluorine and mixtures of oxygen and fluorine.

8. The method of claim 7 wherein said predetermined temperature is the boiling point of nitrogen and said sources of fuel and oxidizer are cooled with liquid nitrogen. l

9. The method of claim 5 wherein said fuel is a hydrocarbon mixture, said predetermined temperature is the boiling point of nitrogen and said sources of fuel and oxidizer are cooled with liquid nitrogen.

10. The method of claim 9 wherein said oxidizer is selected from the group consisting of fluorine and mixtures of oxygen and fluorine.

References Cited UNITED STATES PATENTS 3,009,316 11/1961 McKinley 149-1 X FOREIGN PATENTS 476,227 12/1937 Great Britain. 8,552,000 11/1960 Great Britain.

OTHER REFERENCES Sage, B. H., et al.: Behavior of Hydrocarbon Mixtures Illustrated by a Simple Case, Fourteenth Annular Meeting, American Petroleum Institute, Chicago, 111., Oct. 26, 1933, p. 2 relied on.

Bloomer, 0. T et al.: Physical-Chemical Properties of Methane-Ethane Mixtures, Institute of Gas Technology Research Bulletin #22, p. 2 relied on.

CARLTON R. CROYLE, Primary Examiner.

SAMUEL FEINBERG, MARK M. NEWMAN,

Examiners.

D. HART, Assistant Examiner. 

5. IN METHOD FOR OPERATING A CRYOGENIC PROPELLANT MOTOR WHEREIN A CRYOGENIC LIQUID OXIDIZER AND A CRYOGENIC LIQUID FUEL ARE INJECTED FROM SEPARATE SOURCES INTO A COMBUSTION CHAMBER AND REACTED THEREIN TO PRODUCE HOT COMBUSTION GASES, THE IMPROVEMENT WHEREIN THE SOURCES OF SAID OXIDIZER AND SAID FUEL ARE MAINTAINED AT A PREDETERMINED TEMPERATURE NO GREATER THAN ABOUT -186*C.; SAID OXIDIZER HAS A BOILING POINT BETWEEN ABOUT -186*C. AND SAID PREDETERMINED TEMPERATURE, AND SAID FUEL IS A HIGH DENSITY FUEL HAVING A FREEZING POINT BELOW SAID PREDETERMINED TEMPERATURE. 